1. Field of the Invention
This invention relates to an imaging system and an imaging method.
2. Description of the Related Art
In recent years, lock-in imaging technology for acquiring an image in synchronization with ON/OFF of a light source has been developed. The related art is, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2012-205217 (hereinafter referred to as “Patent Literature 1”) and N. Oda et al., Proceedings of SPIE, Vol. 8496, 84960Q (2012) (hereinafter referred to as “Non Patent Literature 1”).
Patent Literature 1 relates to an imaging device that includes a light source and a camera and is designed so that a measurement object is placed in an optical path therebetween. More particularly, Patent Literature 1 relates to an imaging device for imaging a measurement object in synchronization with a predetermined period of a light source.
As described above, Patent Literature 1 is the imaging device that includes the light source and the camera and is designed so that the measurement object is placed in the optical path therebetween. In this imaging device, the measurement object is imaged in synchronization with the predetermined period of the light source (so-called lock-in imaging), and hence an image resulting from radiation other than the light source can be eliminated and low frequency noise such as l/f noise is cancelled out.
Referring to FIG. 2, a description is given of an example of the lock-in imaging technology described in Patent Literature 1.
A Sync signal 101 determining a frame rate of a terahertz (THz) camera 100 is input to a frequency divider 102 to divide the frequency by 2n (n=1, 2, . . . ). Although not illustrated, the selection of n is switched by a dual in-line package (DIP) switch, for example. A synchronization signal 103 obtained by the frequency divider is input to an AND circuit 106 included in a controller 105 for a THz light source 104 called quantum cascade laser (QCL). AND operation is performed on the synchronization signal 103 and a pulse 108 output from a high voltage pulse power source 107 to produce a QCL drive pulse 109.
The drive pulse 109 is input to a drive circuit 110 so as to irradiate the light from the light source 104 with the predetermined period, thereby irradiating a sample 111 with a THz wave 112. The THz wave 112 reflected on the sample 111 is detected by the THz camera 100 to be converted into an image. Although FIG. 2 illustrates the arrangement in a reflection mode, the lock-in imaging technology is also applicable to the arrangement in a mode in which the THz wave is transmitted through the sample.
Data acquired by the camera 100 is input to an image data acquisition device 113. A CPU 114 included in the image data acquisition device 113 stores in a buffer 115 image data measured in an irradiated period of the THz light source 104, image data measured in an unirradiated period thereof, and difference image data therebetween. A phase compensation circuit 116 compensates for a phase shift caused by a circuit included in the THz camera 100, thereby serving to acquire an image in synchronization with a predetermined timing of the THz light source 104.
FIG. 3 shows a lock-in image 200 acquired in this way.
The lock-in image 200 was acquired in the transmission mode, and the sample 111 was a hair placed under paper. The quantum cascade laser light source 104 having a frequency of 2 THz was irradiated the light with a predetermined period at a lock-in frequency of 3.75 Hz to irradiate the sample 111 with the THz wave 112, and the transmitted THz wave 112 was imaged by the THz camera 100 to acquire the lock-in image 200. Specifications in imaging were four times of frame integration and spatial filtering of 3×3 pixels. An absorption image 201 resulting from the hair is observed. A concentric pattern 202 is considered to result from interference inside the optical system caused by high coherency of the quantum cascade laser.